This invention relates generally to the in situ and non-destructive examination of circumferential surfaces, particularly including welds, and more particularly, obstructed welds. Such surfaces and welds may be found in boiling water nuclear reactors and are particularly associated with the attachment of jet pump diffusers to a baffle plate.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide, sometimes referred to as a grid, is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. The core shroud is a reactor coolant flow partition and structural support for the core components. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. A removable shroud head is coupled to a shroud head flange at the top of the shroud.
A series of jet pump diffusers are typically arranged around the periphery of shroud and attached to a baffle plate (or pump deck) arranged between the shroud support structure and the inner wall of the RPV. In many cases, an adapter ring is provided to ensure a proper fit between the jet pump and the baffle plate, with the various components being fastened with a series of circumferential welds. During operation of the reactor, however, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the jet pump attachment. In particular, lateral seismic/dynamic loading could cause relative displacements at cracked weld locations may produce leakage and misalignment of the core that could comprise the performance of the jet pumps. Given the complex configuration of the attachment between the jet pump and the baffle plate, however, in situ examination of the welds has proven very difficult.
It is desirable, therefore, to provide an apparatus and a corresponding method for inspecting the welds used to attach the jet pump diffuser, the jet pump adapter and the baffle plate that is reliable and is capable of examining the majority of the circumference of at least two circumferential welds without the need for repositioning. When using ultrasonic sensors to examine a weld, the focus point of the ultrasonic beam may be adjusted so that the focal point of the beam aligns with an upper fusion line of the weld and the outer surface of the jet pump tail pipe or the jet pump adapter. The ultrasonic beam may then be repeatedly refocused to move the focal point along the weld fusion line from the outer surface toward the inner surface in discrete increments of about 0.01 to about 0.5 inch. One method for such incremental scanning is disclosed in U.S. Pat. No. 6,332,011, the contents of which are hereby incorporated by reference.
A variety of mechanisms have been devised for the examination of welds, particularly for use in hostile environments such as the interior of RPVs. One such apparatus is disclosed in U.S. Pat. No. 5,568,527, the contents of which are hereby incorporated by reference, and provides a remotely operated apparatus with clamping, sliding, rotational and sensor mechanisms to scan an ultrasonic transducer over specific core spray “T-box” welds including the T-box to cover plate attachment the T-box to thermal sleeve attachment welds.